The problem of maintaining a proper consistency of the constituents in printing in ink drop printing systems, has been recognized in the art. Various schemes have been proposed and used for reconstituting the ink by adding solvents and the like, to compensate for loss of such solvents by evaporation in the printing ink which is collected and returned to the printing ink supply system. Most of the prior approaches either guess at or estimate the amount of solvent to be added from time to time, based upon an estimated exposure of the printing ink to the process of evaporation, or provide complex systems which depend for their operation upon the measurement of some parameter of the printing ink, such as by measuring the velocity of propagation of sound through the material, by measuring the specific gravity, or by measuring the conductivity. Inherently, the latter systems are necessarily complex.
The non-automatic systems, that is the systems which blindly add predetermined quantities of replenisher, have generally failed to take into account the fact that the system may need greater or lesser quantities of replenisher, depending upon the character of the material being printed. During the operation of an ink jet printer, it is known that the volume of ink in the principal supply system will gradually decrease as the ink drops are deposited on the printing receiving medium. Ideally, the volume lost in this matter is compensated for by the addition of fresh ink into the principal ink supply system. Further fluid loss results from evaporation of solvents from the ink. Since the fluid supply systems are generally closed, most of this evaporation occurs between the time that the drops leave the printhead and are caught by the catcher. If no printing were being accomplished, and all of the drops were being caught and returned to the ink system, the fluid loss would be entirely due to solvent evaporation, and the addition of a solvent or suitably constituted replenisher would be appropriate. On the other hand, if all of the drops produced by the printhead were being deposited on a print receiving medium, such as paper, none of the fluid would be lost as a result of evaporation and it would therefore be desirable to make this fluid loss up only by the addition of fresh ink. In actual operation, however, the fluid loss is a combination of both of these factors. Hitherto, blind or mechanical reconstituting systems have been incapable of distinguishing between conditions where the loss of fluid is primarily due to printing or primarily due to evaporation, and hence the more complex systems controlled by parameter measurements have come into being.